One method of cancer diagnosis is a positron emission tomography (PET) examination. In this PET examination, a test agent in which a radioactive substance is bound to a sugar which is likely to gather in cancer cells is administered to a patient. Then, a pair of γ rays (hereinafter, referred to as annihilation γ rays) are detected, which are generated when electron-positron annihilation occurs in the radioactive substance in the test agent gathered in the cancer cells and the generated positrons disappear. In particular, this pair of annihilation γ rays travel in opposite directions by 180 degrees. Therefore, the locations of cancer cells and the like may be identified by detecting the positions of the pair of annihilation γ rays simultaneously incident on the radiation detectors disposed around the specimen, and from the point of view that the radioactive substances exist on the straight line connecting the detection positions, repeatedly measuring this to create a reconstructed image as in a computed tomography (CT).
In such a PET examination device, a large number of detectors are arranged in a space surrounding a patient and identify a detector pair on which the annihilation γ rays are incident. Since the efficiency of radiation detection increases as the number of detectors increases, it is desirable to arrange a large number of detectors. However, as the number of detectors increases, the processing and cost of simultaneous detection events also increase, so that in many cases an inexpensive system using a scintillator and a position sensitive photomultiplier tube is adopted.
Here, in the conventional PET detectors, a large number of detectors are arranged in a ring shape around the patient, but the detectors do not have a resolution with respect to a direction towards the patient (depth direction), so that it is difficult to identify positions with the detectors. Therefore, when the annihilation γ rays are incident on the detectors from an oblique direction, the spatial resolution (position resolution) is reduced.
As a means for solving this, a depth of interaction (DOI) detector has been proposed. In the DOI detector, a plurality of scintillators are arranged in the depth direction, and a measure for distinguishing emission signals from these scintillators is applied, thereby enabling discrimination in the depth direction. Therefore, the spatial resolution of annihilation γ rays incident from an oblique direction may be increased.